Usually ISP (Internet Service Providers) assign you a different IP-address each time you connect to the net via modem, ISDN or xDSL. DynDNS is a service, that provides you with a valid, unchanging, internet domain name (an URL) to go with that (possibly everchanging) IP-address.

This way you have the same name (or URL) to give others when they need to contact your server, regardless of your IP-address. We even provide a service so DynDNS itself can inform your friends if you're not online.